Pregnant women may be exposed to exercise, thermal, or gastrointestinal (hyperemesis) water loss, all of which commonly induce a >10 mOsm increase in plasma osmolality (osm). Although fetal plasma is dependent on maternal osm the impact of maternal dehydration and subsequent rehydration on the fetus has not been explored. Increases in maternal plasma osm well within the physiologic range result in significant fetal endocrine responses (stimulation of arginine vasopressin and renin secretion and perhaps a suppression of atrial natriuretic factor) and subsequent alterations in fetal water dynamics and blood volume. Despite maternal rehydration, fetal endocrine and fluid responses persist beyond a return to basal maternal and fetal plasma osm. As a result, episodic maternal dehydration may contribute to the development of oligohydramnios, premature labor, and/or fetal growth retardation. This project will utilize the chronically catheterized ovine model to investigate the ontogeny of fetal and amniotic fluid water and electrolyte responses to maternal dehydration and subsequent rehydration. A compartmental model will be utilized to quantify alterations in fetal water acquisition (fetal swallowing and transplacental flow) and excretion (urine and lung fluid). Specifically, we have developed and confirmed a method for the measurement of fetal swallowing activity and volume. Fetal urine and lung fluid production will be measured directly, and transplacental flow calculated. The resultant effects on fetal blood volume and the impact on amniotic and allantoic water compartments will be evaluated. The response of selected fetal endocrine systems central to water homeostasis will be examined and the mechanisms of endocrine regulation of fetal fluid and electrolytes and swallowing responses will be studied using selective agonists and antagonists. As maternal and fetal responses to rehydration differ depending upon the mode of hydration, this project also will examine the fetal endocrine and fluid responses to several means of maternal rehydration. Studies of dehydration and rehydration of the ovine model represent a promising approach to understanding maternal-fetal water dynamics, while addressing a potentially critical problem in perinatal medicine.